Electro-hydraulic servo-valves are widely used to control the flow of hydraulic fluid into and out of power cylinders that are used to control, for example, the flow or pressure of fuel, air, or steam. However, some conventional servo-valves do not operate very efficiently or can fail when regulating the flow of contaminated fluids. Further, traditional servo-valves can be adversely affected by vibration. Vibration in the axial direction of the spool can move it from the desired position causing a position error of the actuator. Additionally, conventional servo-valves are typically non-compliant with Div. 1/Zone 1 hazardous location (explosion-proof) requirements. Typically, explosion-proof servo-valves are considered “specials” and are usually expensive devices requiring long lead times.
Further, many conventional servo-valves are two stage using a linear spool type flow control valve as the main fluid flow control element. Generally, tight radial clearances are used to help keep the sliding spool centered within the outer element and reduce fluid leakage. It is possible that these tight clearances can become further constricted if there is contamination in the fluid flow causing the spool to stick, for if the fluid is not filtered to extreme cleanliness levels. The need to maintain tight tolerances leads to time-consuming manufacturing processes and testing of the device, and drives up the cost of traditional servo-valves.
Also, in some cases, contamination in the fluid flow may restrict flow through nozzles in the first stage of the servo-valve. In other cases, small diameter wire used for the torque motor winding is subject to strain failure and is another source of poor reliability. Some conventional servo-valves require hydraulic pressure to move the spool valve to its failsafe position. If the filter supplying the first stage element plugs, or pressure is lost for some other reason, there is a risk that the servo-valve will not easily move to the failsafe position.
In some cases, conventional direct drive or proportional valves (rotary or linear) often have a “wet” drive motor that can collect iron particles from the oil, which can eventually decrease the performance of the valve. Further, conventional direct drive or proportional valves (rotary or linear) that have high flow capacity have high flow forces acting on the moving element. This requires a high-force electrically powered first stage that can have high power requirements and/or be too slow for good dynamic control. Also, conventional direct drive or proportional valves often only have a single control signal input and a single power input. Failure of either input causes the servo-valve to no longer function.
Embodiments of the invention provide an improvement over the state of the art in electro-hydraulic actuators. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.